Tractor-scraper vehicles are used in many construction applications, road building being probably the most common example, to transport heavy loads of material such as dirt, shot rock, gravel or the like from one location to another in conjunction with whatever construction is being carried on, and also for loading, spreading and grading of such material. Typically, tractor-scraper vehicles have a single-axle tractor unit and a single-axle scraper unit, the unit being interconnected by a hitch which allows relative pivotal movement of the units about a vertical axis.
In general, the operators like to "feel" what the vehicle is doing. If the axles of the units are unsuspended, or very stiffly suspended, relative to the frame, this is no problem since the operator will feel every bump that the tractor or scraper hits. However, this results in a rather uncomfortable and tiring ride and therefore shock absorbing axle suspensions are generally used in the tractor unit.
Typically such an axle-suspension system includes a pair of structural links or arms, one on each side of the longitudinal centerline of the vehicle, which interconnect between the axle and frame to hold the axle against forward or rearward movement relative to the frame while allowing the axle to move vertically relative to the frame, and a pair of shock-absorbing devices, one on each side of the longitudinal centerline of the vehicle which interconnect between the axle and frame to hold the frame at a desired level above the axle while permitting vertical movement of the axle relative to the frame in case when the vehicle wheels go over bumps or drop into ground depressions. In tractor units, such shock-absorbing devices commonly include a hydraulic cylinder between the axle and frame and a pressurized accumulator which functions to cause cylinder extension or retraction in response to changes in the loading on the cylinders as the vehicle goes over rough ground.
Such shock-absorbing suspensions introduce "roll-flex" into the system, i.e. an ability of one side of the axle to move up or down relative to the other side, when one wheel of the vehicle hits a bump or drops into a depression.
If the amount of roll-flex permitted by the suspension system is relatively great, the operator will be disturbed because he will not "feel" the bumps or depressions that a wheel of the tractor will hit and will consider that he does not have full control of the machine. The operator will also be disturbed in vehicle operation, after a tractor wheel has hit a bump which is not much noticed by the operator, by the sudden roll or motion introduced into the tractor through the hitch when the corresponding and unsuspended wheel of the scraper hits the same bump.
The amount of roll-flux can be reduced by increasing the stiffness of the structural supports interconnecting the frame and axle to restrain the axle against lengthwise inclination as bumps and depression are encountered by the wheels. However, the greater the amount of such restraint, the more severe are the twisting forces imposed on the structural supports and on their interconnections with the axle and with the frame.
Accordingly, there is a problem of providing an axle suspension system which allows shock-absorbing vertical movement of the axle relative to the frame and in which roll-flex is limited without undue imposition of forces on the structural support for the system.